Decision Feedback Equalization (DFE) is based on the principle that once the value of the current transmitted symbol has been determined, the contribution of intersymbol interference (ISI) to future received symbols can be removed. DFE has a nonlinear feature that is due to a decision device that attempts to determine which symbol of a set of discrete levels was actually transmitted. Once the current symbol has been decided, a filter structure calculates the ISI effect it would tend to have on subsequent received symbols and, thereafter, compensate the input to the decision device for subsequent samples. This post-cursor ISI removal is accomplished by the use of, among other things, a feedback filter structure.
As part of the implementation of DFE in current mode logic (CML), for example, a DFE adaptation is implemented wherein a DFE summing node common mode voltage changes with DFE feedback coefficients. More particularly, because current generally increases as DFE feedback coefficients are increased, so does the common mode voltage. But in such an implementation, the reference signal common mode is fixed thus creating a problem. For example, where an error sense amplifier is typically implemented, the output of such error sense amplifier will not only depend on the differential signal amplitude as desired, but it will also depend on the increased common mode difference, an undesirable effect. In certain situations, such a result impedes or prevents the convergence of DFE adaptation.